SNOM IP phones are known for sending very large packets (1500 bytes) that may be dropped by routers that are connected to PPPOE internet connections. PPPOE internet connections have a maximum packet payload of 1492 bytes, which may not be fragmented by the site router and simply dropped. 

A solution to this is to use a router that will fragment the packet or attempt to forward it as is. Otherwise, non PPPOE internet access must be used if SNOM phones are to be used onsite.