Our company's phone system, or our provider, had a problem with outgoing calls once: you couldn't call users of a certain mobile provider, but only if they were currently using GSM. Anything newer would make it work. The symptom was that even when the cell user picked up, you'd still here the sound like their phone is still ringing. That alone was already mindboggling to me, but the crazy part was that with certain phone models (at our company), the cell user would be able to hear you while you still thought their phone was ringing.