If it happens that it takes a while until the recipient receives your message, either push notifications aren’t working properly on your contact’s phone, or there is a problem concerning background activity. In other words, mobile devices (especially Android devices) restrict the app activity in the background after a while, and the app cannot be reached for calls.
Please ask your contact to follow the troubleshooting instructions for Android (Samsung, Huawei, Xiaomi, Sony, OnePlus, Gigaset, Ulefone Armor, Doogee, etc.) or iOS.
