Had same issue with zoiper. I was thinking that ports on the voip server was closed or not fowarded because the 'I dont hear you' problems are most of the time cause with no port forwarding done right.
If thats not the case because you are using external voip provider then It could be Android related.
LG G2 KOT49I.D80220d 16GB
Rom: CyanogenMod 10.1 by ADC Team Release4 - kernel: 3.4.51 = PhenomKernel v4.3 UV PMEM 400MB Ram - modem: XXKPO - recovery: clockwork-220.127.116.11-ariesve [kernel 2.6] - eMMC:M8G2FA FwRev:0x1f - settings: 245-1804MHz, ONDEMAND/ROW