Hi Philip, I do not understand how lowering the MTU on the WAN interface is going to help. The application server on the carrier end is a Broadsoft SIP server. Here is a excerpt from the Broadsoft documentation about BLF: Depending on the combined length of contact information fields of the users to be monitored, the NOTIFY packets are typically larger than the MTU (maximum transmission unit) defined in the routers, firewalls, and other data transmission equipment. As a result the UDP packets have to be fragmented into a number of smaller packets to fit the MTU size as they are sent out to the network. In Ethernet networks, MTU size is typically set to 1500 bytes. A UDP datagram can be up to 65507 bytes in size. A UDP datagram of 65507 bytes will have to be fragmented into 45 packets ( Max UDP size of 65507 bytes / UDP size per packet of 1480 bytes ~ 45) to be transmitted across a network. With this many number of fragments, customer firewalls or NAT devices may prohibit the entire packet from being delivered to the endpoint. A common issue that has been observed is the firewall or NAT traversing issue. Some firewalls will simply drop any fragmented packets while the others will drop packets when the number of fragmented packets exceeds a configured or default limit. For example, the Cisco ASA 5500 firewall by default will drop a packet if the packet has been fragmented into more than 24 packets. When this happens, phones will not receive the BLF NOTIFY message. As a result phones can only display a certain number of BLF lines. The recommended solution is to configure the firewalls and/or NAT routers at customer premises to handle fragmented UDP packets correctly. These firewall and NAT routers must be configured to support the maximum UDP payload size of 65507 bytes and to allow at least 45 fragmented packets per packet. The Meraki MX64 seems to be only allowing 4 fragmented packets or roughly 5000bytes. I will try your suggestion of lowering the MTU as we are now snookered with this issue, I'll let you know how it goes.
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