Sounds like a one-way audio issue. Because the phones use one protocol for call control (TCP) and a different protocol for the actual voice traffic (RTP/UDP), it is possible (and unfortunately quite common) for a firewall, NAT, or VPN to block or interfere with one (usually the RTP) and allow the other. So you can make and receive calls, but cannot be heard by the far end.

So the question is, where are these phones located and what networking equipment is between them? Make special note of any Network Address Translation and/or any private address space in use.