The “best” secure communications platform depends a lot on your exact environment, but for contractor + field-user communications across mobile and remote users, the strongest options are usually:

Top choice for most orgs: Microsoft Teams + Microsoft 365 security stack

Best when you want:

• Familiar collaboration for contractors
• Mobile access
• Central identity control
• Data loss prevention, conditional access, device compliance, and auditability

Why it’s strong:

• Works well for mixed internal/external users
• Supports guest access and controlled sharing
• Pairs with Entra ID (Azure AD), Intune, Conditional Access, MFA, Sensitivity Labels, DLP, and eDiscovery
• Easier to govern than many “secure messenger” tools at enterprise scale

Tradeoff:

• It’s not the most “air-gapped / ultra-secure messaging” option, and governance must be configured carefully.

Best for high-security communications: Threema Work or Wire

Best when you need:

• Strong end-to-end encrypted messaging
• Minimal metadata
• Simple secure chat and file sharing
• Mobile-first use cases in the field

Why they stand out:

• E2EE by design
• Good for contractor-heavy environments
• More private than mainstream collaboration tools

Tradeoff:

• Less comprehensive than Teams/Slack for workflow, document collaboration, and enterprise integration.

Best for regulated/defense-style environments: BlackBerry SecuSUITE or HCL Domino/Verse with strong controls

Best when:

• You need secure voice, messaging, and hardened mobile communications
• You operate in government, critical infrastructure, or defense-adjacent contexts

Tradeoff:

• More specialized, often more expensive, and sometimes less user-friendly.

Good if you need workflow + security: Slack Enterprise Grid + enterprise security controls

Best when:

• Your contractors already use Slack
• You need channels, integrations, and flexibility

Tradeoff:

• Security is highly dependent on configuration and identity/device controls
• Usually weaker than Microsoft’s full governance stack for compliance-heavy needs

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My practical recommendation

If you want the best balance of security, usability, mobile support, and contractor access, I’d start with:

Microsoft Teams + Entra ID + Intune + Conditional Access + DLP

This is usually the strongest overall enterprise answer.

If your primary need is secure messaging only, choose:

• Threema Work for simple, privacy-focused mobile communications
• Wire if you want more collaboration features with strong encryption

If you need voice + messaging for high-risk operations, consider:

• BlackBerry SecuSUITE

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What “best” should mean for your use case

Look for these features:

• End-to-end encryption or strong enterprise encryption
• MFA and conditional access
• Device compliance / MDM support
• Guest/contractor identity management
• Revocable access when contracts end
• Logging, retention, and audit trails
• File-sharing controls and DLP
• Offline/mobile support
• Voice, chat, photo/video, and location sharing if field crews need them

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If you tell me:

1. your industry,
2. whether you need chat only or chat + voice + file sharing,
3. whether contractors use company-managed devices or BYOD,
   I can recommend a specific platform stack and deployment model.