Without ge-: besucht, verstanden, telefoniert
Verbs with inseparable prefixes such as be-, ver-, er- and ent- omit ge-: besucht, verstanden. Verbs formed with the suffix -ieren also omit it: telefoniert, reserviert. Do not apply this pattern just from the final letters: frieren → gefroren and verlieren → verloren are strong verbs.
Tip
For regular verbs formed with -ieren, replace -en with -t: telefonieren → telefoniert, never getelefoniert. Strong verbs such as frieren and verlieren do not follow this pattern.