GSM is open access
Anyone can get the specs from ETSI, build a device and get it certified. The network provides IP delivery to the Internet (at a price), so any application on any device can connect to any server.
Of course this isn't cheap and easy so the incumbents (whether at chipset or handset level) have an advantage.
Of course there are a whole heap of market shaping the telcos do (handset subsidy, plan pricing, sim locking, etc) but it still remains that I can take any (unlocked) generic GSM phone and use it on any GSM network (having acquired a supported SIM).