This is not accurate. I move countries a lot and change SIMs (and by extension, phone numbers) and the only time recipients note a number change is when I change handsets. As far as I can tell, new phone numbers are stacked upon the existing user ID which is ostensibly based on the handset (likely the IMEI) and nothing much changes to the recipient until that combination is broken.

Weechat's implementation circa last July left a lot to be desired as it couldn't even federate on its own network - messages I received on my iPad were not present on my Android! Perhaps they've addressed this since but (not to be deliberately contrarian) seamless is far from the adjective I'd use; indeed they seemed miles behind their Western contemporaries and more in the league of Line (another large Chinese messenger application for those unfamiliar). I understand they have a huge Asian user-base so there's presumably a gravity factor, but it certainly didn't seem like they had it together at all.